1. Technical Field of the Invention
The present invention relates generally to an improved fuel supply system designed to supply to diesel engines an alternative fuel such as dimethyl ether (DME) which is lower in viscosity and susceptible to vaporization.
2. Background Art
Fuel now being developed as an alternative to diesel fuel such as light oil is showing a tendency to have a lower viscosity and higher evaporability. Typical of such an alternative fuel is dimethly ether which has a cetane number nearly equal to that of the diesel fuel and is lower in concentration of NOx and HC contained in exhaust emissions of the engine. For these reasons, dimethly ether is being expected to become an alternative fuel for diesel engines which produces black smoke in small quantities over the whole of an output range.
Japanese Patent First Publication Nos. 10-281029 and 10-281030 each teach a fuel injection pump for diesel engines employing dimethly ether as a main fuel. The fuel injection pump is designed to have a decreased gap (less than 3 xcexcm) between a plunger and a plunger barrel in order to reduce a leakage of fuel and to return the fuel leaking out of the gap to an inlet pipe of the engine through a leakage gas return line.
A technical paper IMechE (C517/022/96) describes a fuel injection system designed to supply a dimethly ether fuel through a high-pressure fuel supply pump to a common rail and to injectors for realizing low-emission vehicles.
In a case where the gap between the plunger and the barrel in the structure, as taught in each of the above publication Nos. 10-281029 and 10-281030, is 3 xcexcm, this value is typical of standard pumps. Further, returning of the fuel under high pressures to the engine is achieved using a single line. It is, thus, difficult to decrease a leakage of the fuel to a desired quantity. Particularly, under high temperature conditions, the viscosity of the fuel is lowered, thus resulting in a overmuch increase in leakage of the fuel. The total of the fuel leaking out of the fuel injection pump returned to the inlet pipe of the engine and the fuel sprayed from the injectors, thus, exceeds an allowable level, which may result in a difficulty in controlling the diesel engine correctly.
The system, as taught in the technical paper IMechE (C517/022/96), is designed to produce a pressure intermediate between an injection pressure and a fuel feed pressure to avoid evaporation of the fuel and returns a leaking fuel to a fuel path kept at the intermediate pressure. The injector is equipped with a solenoid valve. A difference between the intermediate pressure and the injection pressure is used to open the injector. This eliminates a leakage of the fuel from the injector to a lower-pressure side completely, but however, resulting in an overmuch leakage of the fuel from high to intermediate pressure side.
Further, the solenoid valve built in the injector is of a hydraulically servo type and thus complex in structure, which result in a difficulty in ensuring a quick response.
The production of the intermediate pressure requires a large number of control valves and intermediate tanks, thus resulting in a difficulty in using the system with diesel engines.
The system is also designed to relieve the fuel from the feed tank, the intermediate tank, and the common rail to a purge tank using a solenoid valve during standstill of the engine. This, however, requires a supply pump to output a large quantity of fuel when it is required to fill the common rail with the fuel, thus resulting in a time delay in starting the engine.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide an alternative fuel supply system which has a simple structure designed to reduce the quantity of fuel leaking out of a high-pressure pump and return it to a fuel tank, thereby ensuring a desired quantity of fuel to be fed to an internal combustion engine, which results in improved fuel economy, engine startability, and engine drivability.
According to one aspect of the invention, there is provided a fuel supply apparatus for supplying fuel such as a dimethly ether used as an alternative to a typical fuel for an internal combustion engine. The fuel supply apparatus comprises: (a) a high-pressure pump working to feed a fuel which is susceptible to vaporization to injectors from a fuel tank through a common rail under a given high pressure; (b) a return line extending from the high-pressure pump to the fuel tank; and (c) a pressurizing mechanism working to add to the return line a back pressure higher than a vapor pressure of the fuel. The addition of the back pressure higher than the vapor pressure of the fuel facilitates liquidizing of the fuel. When the fuel is injected into the injectors, a high-pressure feed pressure and an injection pressure are held within the closed circuit in a fluid-tight fashion without use of an intermediate pressure. The fuel leaking from the high-pressure pump at a lower pressure is introduced into a purge tank or an inlet pipe of the engine, thereby resulting in a decrease in leakage of the fuel, which improves the efficiency of burning of the fuel and ensures the startability and drivability of the engine. In a case where a dimethly ether which is susceptible to vaporization and has a low viscosity is used, exhaust emissions of the engine contains less NOx, HC, and black smoke.
In the preferred mode of the invention, the high-pressure pump includes a pump chamber to which the fuel from the feed pump is inputted through a fuel gallery at a feed pressure, a plunger reciprocating within a plunger chamber formed within a cylinder to pressurize the fuel within the pump chamber and inject the pressurized fuel into the engine through a discharge valve and each of the injectors, a higher-pressure leakage line, and a lower-pressure leakage line. The higher-pressure and lower-pressure leakage lines work to withdraw the fuel leaking out of the pump chamber through a gap between the plunger and the plunger chamber following reciprocating motion of the plunger.
The higher-pressure leakage line communicates with the fuel gallery through a feed line. The lower-pressure leakage line communicates with a sealed chamber formed beneath the plunger.
The fuel flowing through the lower-pressure leakage line is withdrawn through a withdrawing means provided in the cylinder.
The withdrawing means is implemented by a leakage path formed in a side wall of the cylinder.
The apparatus further comprises a solenoid valve which is disposed within an upper end portion of the cylinder and includes a valve member, a valve body, a non-magnetic member, and a plurality of cores. The valve member works to selectively open and close a line connecting between the pump chamber and the fuel gallery to control an amount of the fuel flowing out of the discharge valve. The valve body is disposed within the upper end portion of the cylinder in a fluid-tight fashion. The cores are arranged coaxially with each other through the non-magnetic member and welded rigidly in a fluid-tight fashion inside the solenoid valve.
The discharge valve is opened in response to a pressure which acts thereon and is lower than the feed pressure.
The apparatus further comprises a pressure regulator connected to the fuel gallery. The pressure regulator includes a valve which is urged by a spring into a closed state. The valve of the pressure regulator is moved by a back pressure arising from a vapor pressure of the fuel against a mechanical pressure produced by the spring so that the valve is opened to discharge the fuel flowing from the fuel gallery.
The apparatus further comprises a pressure sensor and a temperature sensor which measure a pressure and a temperature of the fuel whose bulk modulus of elasticity and density are sensitive to changes in pressure and temperature of the fuel and a control circuit which works to correct a discharged amount of the fuel from the high-pressure pump as functions of the pressure and temperature measured by the pressure and temperature sensors to modify a pressure of the fuel stored in the common rail to a given level and to control a quantity of the fuel injected to the engine through the injectors to a give value as a function of an operating condition of the engine.
The pressurizing mechanism may be implemented by a compressor which works to pressurize the fuel being returned to the fuel tank.
According to the second aspect of the invention, there is provided an alternative fuel supply apparatus for an internal combustion engine which comprises: (a) a high-pressure pump connected to a fuel tank through a feed pump, a fuel cooler, and a fuel filter, the fuel tank storing therein a vaporable fuel under pressure higher than a vapor pressure of the fuel; (b) a common rail within which the fuel supplied from the fuel tank through the high-pressure pump is stored at a given high pressure, the common rail being connected to injectors working to inject the fuel into the engine; (c) a pressure limiter installed in the common rail to set a maximum pressure within the common rail; (d) a fuel return line returning fuel relieved from the common rail through the pressure limiter and fuel flowing out of the high-pressure pump to the fuel tank; and (e) a fuel selector valve, a purge tank, a compressor, and a fuel cooler installed in the fuel return line in this order. This structure holds a high-pressure feed pressure and an injection pressure within a closed circuit in a fluid-tight fashion without use of an intermediate pressure when the fuel is injected into the injectors. The fuel leaking from the high-pressure pump at a lower pressure is introduced into the purge tank or an inlet pipe of the engine, thereby resulting in a decrease in leakage of the fuel, which improves the efficiency of burning of the fuel and ensures the startability and drivability of the engine.
In the preferred mode of the invention, the high-pressure pump includes a pump chamber to which the fuel from the feed pump is inputted through a fuel gallery at a feed pressure, a plunger reciprocating within a plunger chamber formed within a cylinder to pressurize the fuel within the pump chamber and inject the pressurized fuel into the engine through a discharge valve and each of the injectors, a higher-pressure leakage line, and a lower-pressure leakage line. The higher-pressure and lower-pressure leakage lines work to withdraw the fuel leaking out of the pump chamber through a gap between the plunger and the plunger chamber following reciprocating motion of the plunger.
The higher-pressure leakage line communicates with the fuel gallery through a feed line. The lower-pressure leakage line communicates with a sealed chamber formed beneath the plunger.
The fuel flowing through the lower-pressure leakage line is withdrawn through a withdrawing means provided in the cylinder.
The withdrawing means may be implemented by a leakage path formed in a side wall of the cylinder.
The apparatus further comprises a solenoid valve which is disposed within an upper end portion of the cylinder and includes a valve member, a valve body, a non-magnetic member, and a plurality of cores. The valve member works to selectively open and close a line connecting between the pump chamber and the fuel gallery to control an amount of the fuel flowing out of the discharge valve. The valve body is disposed within the upper end portion of the cylinder in a fluid-tight fashion. The cores are arranged coaxially with each other through the non-magnetic member and welded rigidly in a fluid-tight fashion inside the solenoid valve.
The discharge valve is opened in response to a pressure which acts thereon and is lower than the feed pressure.
The apparatus further comprises a pressure regulator connected to the fuel gallery. The pressure regulator includes a valve which is urged by a spring into a closed state. The valve of the pressure regulator is moved by a back pressure arising from a vapor pressure of the fuel against a mechanical pressure produced by the spring so that the valve is opened to discharge the fuel flowing from the fuel gallery.
The pressure limiter sets a maximum pressure within the common rail and is controlled in a fluid-tight fashion by opening and closing a solenoid valve.
The feed pump is installed in the fuel tank which is kept at a vapor pressure of the fuel. The vapor pressure of the fuel is added to a feed pressure of the feed pump.
The fuel selector valve may be implemented by a three-way valve. When the fuel returning from each of the high-pressure pump and the pressure limiter is in a gaseous state, the fuel selector valve feeds it to the purge tank. When the fuel returning from each of the high-pressure pump and the pressure limiter is in a liquid state, the fuel selector feeds it to the fuel tank.
The compressor works to pressurize the fuel which is stored within the purge tank in a gaseous state and bring it into a liquid state through the fuel cooler which is, in turn, returned back to the fuel tank.
The high-pressure pump has a discharge valve which is opened in response to a pressure lower than or equal to the sum of a feed pressure of the feed pump and a vapor pressure of the fuel within the fuel tank. At start-up of the engine, a vaporized fuel is withdrawn from the discharge valve to the fuel tank in a liquid state through the common rail, the pressure limiter, the fuel selector valve, and the purge tank.
The apparatus further comprises a pressure sensor and a temperature sensor which measure a pressure and a temperature of the fuel whose bulk modulus of elasticity and density are sensitive to changes in pressure and temperature of the fuel and a control circuit which works to correct a discharged amount of the fuel from the high-pressure pump as functions of the pressure and temperature measured by the pressure and temperature sensors to modify a pressure of the fuel stored in the common rail to a given level and to control a quantity of the fuel injected to the engine through the injectors to a give value as a function of an operating condition of the engine.